Polyimide has been widely applied to various electronic parts because of its excellent heat resistance, electrical characteristics and mechanical characteristics. For rationalization of the operation, it has hitherto been attempted to endow polyimide with photosensitivity.
Conventional proposals with respect to positively photosensitive polyimide include a method of mixing a polyimide precursor with a photosensitive orthonaphthoquinone diazide compound as disclosed in JP-A-52-13315 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and a method of mixing a soluble polyimide with a photosensitive orthonaphthoquinone diazide compound.
These known photosensitive polyimide compositions are usually coated as a solution on a support, dried, and then irradiated with actinic ray, such as ultraviolet light, through a photomask. The exposed area is then removed by dissolving with an arbitrary developer to thereby form a desired image. After image formation, the image area is subjected to a high temperature heat treatment to cyclize the polyimide precursor, followed by removal of the remaining solvent, water produced by cyclization, naphthoquinone diazide, etc. by vaporization to obtain a heat resistant polyimide film.
However, since the above-described conventional photosensitive compositions generally comprise a mixture of a polyimide precursor or polyimide and a photosensitive compound, i.e., orthonaphthoquinone diazide, exposure to light only results in solubilization of orthonaphthoquinone diazide in an alkali, while the structure of the polymer skeleton of polyimide receives no influence from light and remains unchanged.
Therefore, orthonaphthoquinone diazide must be used in a large proportion in order to assure resistance of the unexposed area against dissolution with a developer. This leads to volumetric shrinkage on curing, thus deteriorating dimensional precision.